Conventionally, for stabilizing the extremely soft soil, an upright or vertical hole is formed in the soil using a digging device mounted on a base machine and the soil dug by the digging device is agitated and mixed with a hardening agent so as to harden and thereby stabilize the soil.
In the above soil stabilizing operation, the soil agitation and mixing operation is carried out by agitating bars or the like which are fixedly mounted on a rotary shaft or an auger of the drilling device.
In such agitating structure, however, since all the agitating bars rotate along with the rotary shaft, the soil dug by the agitating structure also rotates along with the rotating shaft in the same direction so that the soil cannot be agitated sufficiently.
Especially in a soil stabilizing operation using a powdery hardening agent in and made of extremely soft soil such as muddy soil which contains a considerable amount of water, the soil dug by the agitating structure adheres around the agitating bars and forms a spherical soil ball and the soil agitating and mixing operation becomes entirely impossible.